1. Field of the Invention
This invention relates generally to wireless communications devices and, more particularly, to a system and method for making a determination of whether to stay with an existing wireless communications coverage network.
2. Background
A key concern for wireless communications devices (referred to herein interchangeably as a mobile handset) is acquiring coverage with the highest probability of providing a good grade of service. Within a cellular network, often a mobile handset finds itself in a coverage zone on an edge of a cell site, often characterized by a low power limit on the forward link (i.e., transmission from the cell site or base station), and sometimes a non detected reverse link (i.e., transmission from the mobile handset). This coverage zone is sometimes referred to as the Grey Zone.
To better understand the problems associated with the Grey Zone, FIG. 1A illustrates a mobile handset 100 operating in a cellular network with at least one base station 105. Base station 105 transmits a signal represented by signal strength 110 that diminishes log-normally as a function of distance. FIG. 1A shows “distance from base station” plotted along the horizontal axis. However it will be understood by those of skill in the art that other factors besides distance to the base station 105 effect the signal strength received by the handset. For example, an object such as a hill, or a car may effect the signal strength. As another example, variations in air density can effect the signal strength. Distance from the base station 105 is used herein as a shorthand for all of the cases that can effect the signal strength.
At some position 115, the mobile handset 100 may be too far from the base station 105, such that the base station 105 cannot detect the mobile handset's 100 transmission. The base station 105 is less limited in the power it can transmit than the mobile handset 100 and therefore can send a strong enough signal that the mobile handset 100 can detect, but at the same time the base station 105 may not detect a signal from the mobile handset 100. In this situation where only the base station 105 is transmitting detectable signals, the mobile handset 100 cannot make a call. This area is called the Grey Zone.
To overcome this problem, current wireless standards implement a strict threshold, that if met, switches the mobile handset 100 from the current network (i.e., base station 105) in search of another network. For example, the Mobile Station-Base Station Compatibility Standard for Dual-Mode Spread Spectrum Systems (commonly know as TIA/EIA-95-B) uses two thresholds: the Ec threshold and Ec/Io threshold, to determine when to seek an alternative network. The Ec threshold and Ec/Io threshold stand for the pilot power threshold and the pilot Ec/Io threshold respectively. See TIA/EIA-95-B at Sec. 6.6.2.2.5. In practice, the mobile handset 100 determines the pilot signal strength and the signal-to-noise ratio of the pilot signal (the SNR) from the signal levels transmitted by the base station 105. The mobile handset 100 also receives a data message from the base station 105 that includes the Ec threshold and the Ec/Io threshold set by the network.
The mobile handset 100 compares the pilot signal strength to the Ec threshold and the SNR to the Ec/Io threshold. In one implementation, if either exceeds the threshold, the mobile handset 100 will seek an alternate network. In another implementation, if both thresholds are exceeded, the mobile handset 100 will seek an alternate network. The strict threshold comparison effectively creates a threshold 120, such that when the mobile handset 100 crosses to the right of the threshold 120, the mobile handset 100 switches from the existing network and seeks alternate coverage.
The problem with this strict threshold comparison method is that it may result in switching from a base station 105 prematurely. Because the pilot signal and the SNR are both functions of position, among other factors, and a mobile handset 100 can change position rapidly, using a strict comparison to thresholds could result in changing base stations too rapidly, and potentially degrading the quality of service. To illustrate, consider a mobile handset 100 traveling on a windy road that at time one is at distance 125. At some short time later, the mobile handset 100 is at distance 130. As it continues is travel, the mobile handset 100 vacillates between distance 125 and distance 130. Both the pilot signal and the SNR for distance 125 may favor base station 105, while the both pilot signal and SNR for distance 130 exceed the relevant thresholds (i.e., threshold 120). The mobile handset 100 using a strict threshold comparison would switch from the base station 105 upon determining that the thresholds were exceeded, and the mobile handset 100 would seek alternative service. This switch may result in diminished service, if for example, alternate service is not available to the mobile handset 100.
Additionally, as mentioned above, the RF level is dynamic and can often oscillate by several dB even when the mobile handset 100 is in a stationary position. The oscillations in the RF level are caused by environmental effects in the area. As a result, even if the mobile handset 100 is positioned on the left side of point 120 in FIG. 1A the received signal strength may cross below the Ec threshold or the Ec/Io threshold at a short distance from the point 120 on FIG. 1A. The hard decision threshold will cause the mobile handset 100 to switch current networks prematurely and results in diminished service.
A similar problem exists when a mobile handset 100 is caught between two competing base stations. FIG. 1B illustrates mobile handset 100 in a network that includes at least two base stations 105 and 135. As discussed above, base station 105 transmits a signal represented by signal strength 110, and similarly, base station 135 transmits a signal represented by signal strength 140. Both signal strengths 110 and 140 diminish log-normally as a function of distance.
Most current methods use the SNR to select between base station 105 and base station 135. For example, if the mobile handset 100 were located at distance 145 (i.e., significantly, closer to base station 105 than base station 135), then the SNR would clearly favor base station 105 and the mobile handset 100 would select base station 105. Similarly, at distance 150, the mobile handset 100 would select base station 135.
As with the situation described above with reference to FIG. 1A, because the SNR is a function of position, among other factors, and a mobile handset 100 can change position rapidly, using strictly a comparison of SNR of competing base stations 105 and 135 could result in changing base stations too rapidly, and potentially degrading the quality of service. To illustrate, consider a mobile handset 100 traveling on a windy road that at time one is at distance 155. At some short time later, the mobile handset 100 is at distance 160. As it continues its travel, the mobile handset 100 vacillates between distance 155 and distance 160. The SNR for distance 155 may favor base station 105, while the SNR for distance 160 may favor base station 135. The mobile handset 100 would then constantly switch between the two base stations. The constant switching may result in occupying additional resources, increased standby time, and other problems, ultimately decreasing service quality. This possibility must be weighed against the benefit of the slightly more beneficial SNR. When, as in distance 155 and distance 160, the SNR difference is likely very small, the possible costs of switching outweigh the benefits.
To overcome this potential over-switching problem, it is known in the art to not simply compare the SNRs, but rather to compare the SNRs plus some Delta. For example, if the mobile handset 100 is at distance 160 and currently using base station 105, the mobile handset 100 would change to base station 135 if and only if:SNR(at base station 105)+Delta<SNR(at base station 135)
Thus, the Delta makes it preferable to stay on the existing base station, unless and until the signal quality drops to a point where it is clearly less desirable than another received base station signal. The Delta effectively sets up two thresholds 165 and 170, where the mobile handset 100 will always select base station 105 if the mobile handset 100 is located to the left of threshold 165 and base station 135 if located to the right of threshold 170. If the mobile handset 100 is located between thresholds 165 and 170, the mobile handset 100 will not switch.
At least two significant problems exist with the SNR comparison method described above and illustrated in FIG. 1B. The first problem is that the SNR comparison, even with the Delta, may result in switching base stations (either 105 or 135) too often. As illustrated above, the mobile handset 100 may be on a winding road that may take to a distance 150, for a very short time, whose SNR (plus the Delta) favors switching the base station, from say base station 105 to base station 135. But the mobile station 100 winds back to a distance 175 where the SNR favors the base station 105, again causing a switch. Despite the fact that the mobile handset 100 only went to distance 150 briefly, the mobile handset 100 still made the switch. This is similar to the premature switching problem described with reference to FIG. 1A.
A second problem is that the mobile handset 100 may not switch to the most favorable base station (either 105 or 135) when in the area between the thresholds 165 and 170. For example, as described above a mobile handset 100 currently using base station 105 may move to distance 160 and come closer and closer to threshold 170, but never crossing threshold 170. At a position approaching threshold 170, it would be advantageous to switch to base station 135. The method described above, however would not switch to base station 135, but would remain with base station 105 because the SNR of base station 135 would not overcome the SNR, including the Delta, of base station 105. Thus, the mobile handset 100 would not operate with the most favorable base station that yields the best service.
Given the problems detailed with reference to FIGS. 1A and B, it would be advantageous if a mobile handset 100 could accurately determine when to exit the existing coverage network so as ensure best possible service and to avoid premature or late exits.